This invention relates to ink-jet printing, and is more particularly concerned with color printing using a combination of slow- and fast-drying inks to produce high quality images without intercolor bleeding.
In existing thermal ink jet printing, the printhead typically comprises one or more ink ejectors, such as disclosed in U.S. Pat. No. 4,463,359, each ejector including a channel communicating with an ink supply chamber, or manifold, at one end and having an opening at the opposite end, referred to as a nozzle. A thermal energy generator, usually a resistor, is located in each of the channels, a predetermined distance from the nozzles. The resistors are individually addressed with a current pulse to momentarily vaporize the ink and form a bubble which expels an ink droplet. As the bubble grows, the ink rapidly bulges from the nozzle and is momentarily contained by the surface tension of the ink as a meniscus. As the bubble begins to collapse, the ink still in the channel between the nozzle and bubble starts to move towards the collapsing bubble, causing a volumetric contraction of the ink at the nozzle and resulting in the separation of the bulging ink as a droplet. The acceleration of the ink out of the nozzle while the bubble is growing provides the momentum and velocity of the droplet in a substantially straight line direction towards a print sheet, such as a piece of paper. Because the droplet of ink is emitted only when the resistor is actuated, this type of thermal ink-jet printing is known as "drop-on-demand" printing. Other types of ink-jet printing, such as piezoelectric, continuous-stream, or acoustic, are also known, and are also applicable to the present invention.
In a single-color ink jet printing apparatus, the printhead typically comprises a linear array of ejectors, and the printhead is moved relative to the surface of the print sheet, either by moving the print sheet relative to a stationary printhead, or vice-versa, or both. In some types of apparatus, a relatively small printhead moves across a print sheet numerous times in swaths, much like a typewriter; alternatively, a printhead which consists of an array of ejectors and extends the full width of the print sheet may be passed once down the print sheet to give full-page images, in what is known as a "full-width array" (FWA) printer. When the printhead and the print sheet are moved relative to each other, imagewise digital data is used to selectively activate the thermal energy generators in the printhead over time so that the desired image will be created on the print sheet.
With ink-jet printing, it is also possible to create multicolor images on a print sheet. This type of printing may be used for full-color images, such as to reproduce a color photograph, or can be employed for "highlight" color, in which colored additions are made to a main portion of the image or text, which is typically black. In either case, the most common technique for color ink jet printing has been to sequentially image two or more colors, in separate printing steps, onto the single print sheet. This superimposition can be carried out in any number of ways. To take the example of a full-width apparatus printing black and one highlight color, an apparatus may print out the entire black portion of the desired highlight image on the sheet, and then recirculate the print sheet once again to image the highlight color portion of the image onto the same sheet from another printhead loaded with the colored ink; such a system has a serious disadvantage in the production of accurate registration of the composed images. Alternately, two printheads may be positioned very close to each other, and render the two portions of the image onto the print sheet almost simultaneously, although two different areas of the print sheet will be printed upon by the different printheads at the same time or with a small time lag. For a full-color process image, four types of ink (yellow, magenta, cyan, and black) are emitted from four separate printheads during printing as the print sheet is moved relative to them. Contouring of four-color images, e.g. for the reproduction of color photographs, are typically created by "superpixeling" or "dot-next-to-dot" (DND) techniques, in which each pixel of the image is created from three or more partially-overlapping spots of different colors, as described, for example, in U.S. Pat. No. 4,999,646.
In any ink-jet printing apparatus, but particularly in color-printing applications, a key concern is the rapid and efficient drying of the ink which has been placed on the print sheet by the printheads. If wet ink is allowed to remain on the print sheet for an appreciable length of time, the image is likely to smear as the print sheet continues on its path through the apparatus. In color ink jet printing situations, another important problem related to ink drying is known as "intercolor bleed." This is a bleeding of one color portion of the image into another portion of the neighboring image of different color. This becomes most apparent when black image is imaged immediately adjacent to an area printed with a color such as cyan, magenta, or yellow. In such a case, the black ink will be seen to bleed into the color area (e.g., cyan, magenta, and yellow) to create a conspicuous print defect. If a composite color is made in the color area (e.g. by combining cyan and magenta to make a shade of blue), the problem will be particularly acute because of the large amount of liquid on the sheet surface. The lighter colored ink will bleed into the black portions of the image as well, but bleeding in this direction will not be as noticeable.
To avoid the print defects caused by inefficient drying of ink on the substrate and intercolor bleed, one solution is to employ either "quick penetrating" inks or special printing techniques. The quick penetrating inks have rapid drying properties, and penetrate deeply into the paper fibers soon after their ejections onto the print sheet. It is intended that, in the color context, the ink of one color will have dried before the imaging of the next color in the printing process, which may mean that the ink will be expected to dry within a small fraction of a second. Another problem with quick-penetrating inks in plain paper printing is known as "feathering," in which the ink is absorbed into the paper so quickly that traces of the ink appear to "feather out" around the areas on the print sheet where the ink was intended to be placed. This print defect may be tolerable in certain full-color images, but is certainly unacceptable in the printing of high quality text.
Co-pending U.S. patent application No. 07/975,027 (now with allowed claims), filed Nov. 12, 1992, and entitled "Thermal Ink-Jet Printing With Fast- and Slow-Drying Inks," discloses an ink-jet printing apparatus in which the problems of bleeding or feathering are avoided by providing two black inks. A slow-drying black ink is used in some portions of the image and a relatively fast-drying black ink is used for other portions of the image. Basically, the slow-drying black ink is used where the black portion of the image does not abut areas printed with other colors of the sheet, such as in the text of a document. The fast-drying black ink is used in situations where the black ink is placed in close proximity to inks of other colors on the sheet, particularly in reproducing a color image or photograph. In a preferred embodiment of the invention, the slow-drying black ink is first placed by itself on the sheet, and then immediately thereafter heat is applied to the sheet to dry the relatively slow-drying ink. Only after the slow-drying ink has substantially dried on the sheet is the fast-drying black ink, along with the other comparably fast-drying colored inks, placed on the sheet. In this way, a compromise is effected, using a slow-drying, non-feathering black ink in text, where such feathering would be conspicuous, and then using the fast-drying black ink in situations where intercolor bleed with other abutting colors would be of concern.
Co-pending U.S. patent application No. 08/226,582 filed Apr. 12, 1994, discloses a method of printing a desired image based on digital data on a substrate. A subset of the digital data relating to a portion of the desired image is identified, wherein pixels associated with the digital data are of a predetermined spatial relationship with pixels intended to be of a different color as the pixels associated with the digital data. The subset of digital data is caused to activate a printhead adapted to emit ink of a first type. Data not of the subset of digital data is caused to activate a printhead adapted to emit ink of a second type, the ink of the first type being adapted to dry at a faster rate than the ink of the second type.
There is therefore still a need to provide an ink-jet color printing apparatus that includes printhead structures and electronic control subsystems for using fast- and slow-drying inks in advantageous manners to produce high quality black and multi-color images without intercolor bleeding. Some attempts to provide ink jet printers of the sort, for example, are further described below as references.
U.S. Pat. No. 4,234,775 discloses the concept of using applied microwave energy to remove moisture from a moving web.
U.S. Pat. No. 4,599,627 discloses an ink-jet printer which employs a writing fluid having two components. One component is located in a separate reservoir, which feeds into a second printhead which is disposed farther from the sheet than the first printhead. The second printhead is mounted on the same carriage with the printhead in an offset position, and ejects the second component onto locations on the medium corresponding to the printed characters made by the printhead. The use of two fluid components achieves a reduced drying time and eliminates clogging of the printhead.
U.S. Pat. No. 4,833,491 discloses a color ink-jet printer having an ink delivery system adapted for multicolor images. When a given printing mode, such as highlight color, is selected, ink supply systems associated with particular ink cartridges are purged and re-connected as needed to print particular colors.
U.S. Pat. No. 4,943,813 discloses various methods of using an ink-jet device for creating overhead transparencies. In order to avoid certain common problems associated with this process, the patent discloses techniques such as printing a first spot of color at a selected location in a first pass of an ink-jet printer, and printing a second spot of color at the same location in a second pass; printing alternate spots of color in successive scans; and printing alternate lines of color in successive passes.
U.S. Pat. No. 4,952,942 discloses a color ink-jet printing method using inks of two or more colors including yellow. Mixed color areas containing yellow are created by using a plurality of inks with different dye concentrations for at least one color, wherein the yellow ink is attached onto the recording medium always prior to attachment of other inks onto the medium.
U.S. Pat. No. 4,982,207 discloses a heater construction for an ink-jet printer having a rotary print platen for holding a print sheet. The platen heater includes a hollow shell having vacuum holes for sheet attachment. A heating foil is mounted in heat transfer relation with the interior of the shell and is coupled by brush contacts to an electrical power source.
U.S. Pat. No. 4,999,646 discloses a full-color ink-jet printing technique in which contoured four-color images, such as in color photographs, are created by "superpixeling" or "dot-next-to-dot" (DND) techniques, in which each pixel of the image is created from three or more partially-overlapping spots of different colors.
U.S. Pat. No. 5,124,719 discloses an ink composition including a first component, a second component having a higher melting point than the first component, and a colorant. During printing, the first component is intended to permeate into the recording medium. As the concentration of the first component decreases and the temperature of the ink cools, the second component with the colorant will solidify and the first component will act to fix the second component and colorant to the medium.
U.S. Pat. No. 4,682,216 discloses a method of forming a full-color image. When a black component of a color pictured data is smaller than a predetermined value, the black component is expressed by using the combination of three colored inks. When the black component is larger than the predetermined value, the black area is rendered by using alternately a black ink and the three color ink combination.
U.S. Pat. No. 4,467,348 Fujii et al. discloses a recording (printing) apparatus for printing on a sheet first with black ink (printhead 125B), then with red ink (printhead 125R) as the sheet is moved in a process direction R. An air fan 126, a means shown for drying the printed two color image, is mounted downstream of both the first printhead 125B and the second printhead 125R.
U.S. Pat. No. 4,672,432 Sakurada teaches a particular configuration of a multicolor ink jet printing apparatus for reproducing halftone color images in which each picture element is comprised of a plurality of dots or cells. Some of the picture elements may contain dots of different colors and optical densities. The apparatus of Sakurada includes a bunched up plurality of printheads some containing inks exhibiting high optical densities, and some containing inks exhibiting low optical densities. In printing, where there is more than one combination of ink colorants representing the density of a picture element, the colorant having the largest number of dots is selected for printing. More colorants of different densities are used for cyan and magenta inks, than are used for black and yellow inks.
U.S. Pat. No. 5,156,675 Breton et al., teaches particular compositions per se of fast-drying ink inks. Many such compositions per se are known in the art.